Climate change poses a serious threat to coastlines, bringing rising sea levels, warming waters, and changing storm patterns. Coral reefs, mangroves, oyster reefs, and salt marshes can help reduce the risk of coastal hazards to coastal communities
Coastal protection from natural coastlines
- Coral reefs reduce wave energy by up to 97 percent.
- In the Philippines, Mexico, and Malaysia, the annual expected coastal protection benefits of reefs exceed $450 million per year, per country
- A 100-meter-wide belt of mangroves can reduce wave heights between 13 and 66 percent, and up to 100 percent where mangroves reach 500 meters or more in width.
- Saltmarshes can attenuate as much as 50% of smaller waves even by a barrier of just 10 meters
Coral Reefs’ Sea Defense Value
In a first rapid assessment to show the relative value of reefs as sea defenses worldwide, we first mapped the areas threatened either by erosion and/or inundation. The resulting map clearly shows that many coral reefs—being far from people—are of zero value in coastal defense. By contrast the highest values are of course closest to areas of high human population densities. These do not always correlate with the healthiest coral reefs, or the most important reefs for other values such as tourism or fish production, however their importance in coastal protection is critical and should provide an important warning for decision-makers regarding the protection and improvement of water quality as a means to ensure the continuation of this ecosystem service.
Flood Protection Benefits of Coral Reefs
To further assess the benefits of coral reef protection, TNC has worked with collaborators at the Institute of Environmental Hydraulics at the University of Cantabria (IH Cantabria) and UC Santa Cruz to combine ecological, engineering and economic approaches to value the benefits of reefs for flood reduction to people and property. View interactive map
Using high resolution modelling of flood hazards and damages, we have estimated the expected benefit of coral reefs for storm flood reduction. Building on recommended approaches, we looked at the potential damage costs from four different storm return periods (one-in-10 year, one-in-25, -50, and -100-year storms). In each case, we compare flooding for scenarios with reefs at their present height with scenarios for a loss of one meter in reef height. We estimate the land, population and built capital ($) flooded across all coral reef coastlines to a 90 meter resolution (Figure 1). We examined flooding in cross-shore profiles every two kilometers for all coral reefs globally and summarized these in coastal units of 10 profiles, giving our summary coastal segments of approximately 20-kilometer stretches, across >71,000 kilometers of coastline with coral reefs. For each coastal segment, we then derive the expected benefit of coral reefs for flood damage reduction from local to global levels in social and economic terms.
What we found is that reefs provide significant flood protection savings for people and property with some of their most important flood protection benefits from the most frequent storms. Just small declines in the height
of the reef crest allow much more wave energy to pass through to flood coastlines. For one-in-10-year events, storm costs would more than triple if we lost just one meter in the height of reefs. Reefs provide significant benefits even for higher intensity, 100-year events where damages would increase to $219 billion if we do not manage reefs well.
The countries with the most to gain in annual benefits from reef conservation and restoration include Indonesia, Philippines, Malaysia and Mexico. For each of these countries, the annual expected benefits of reefs exceed $450 million per year. And this benefit is only from the topmost meter of reefs and for direct flood reduction to built capital. Reefs provide many other benefits and the effects of flood protection on people are widely felt across countries and economies. The countries that may see the greatest annual benefits relative to their GDP include many Small Island Developing States particularly across the Caribbean.
Mangrove Coastal Protection: Guidance for Coastal Managers and Policy Makers
The Mapping Ocean Wealth team undertook detailed reviews of all the existing research into the role of mangroves in coastal protection. The resulting publication describes how a 100-meter-wide belt of mangroves can reduce wave heights between 13 and 66 percent, and up to 100 percent where mangroves reach 500 meters or more in width. If mangrove forests are sufficiently large, they can reduce storm surge peak water levels between 4 and 48 centimeters per kilometer of mangrove. In low-lying areas, even such relatively small reductions in peak water levels can reduce flooding, and prevent property damage.
Mangrove Coastal Protection in the Philippines
The Nature Conservancy, in partnership with the World Bank, The Philippines government, and the Environmental Hydraulics Institute IHCantabria, recently completed a study that quantifies the risk reduction benefits from mangroves in the Philippines.Using high-resolution flooding models, the Technical Report examines the flooding that would occur with and without mangroves under different storm conditions throughout the Philippines, and estimates the annual expected benefits of mangroves for protecting people and property in social and economic terms.
The Report, and its accompanying Policy Brief, provide a social and economic valuation of the flood protection benefits from mangroves in the Philippines. The work aims to support decisions across development, aid, risk reduction and conservation sectors as they seek to identify sustainable and cost-effective approaches for risk reduction. View the map associated with this project here.
Other MOW-linked studies look at the costs of natural versus engineered sea defenses and show that restoring mangroves can be two- to five-times cheaper than building a concrete breakwater to provide the same degree of protection.
In Mobile Bay, Alabama, large-scale efforts are now underway to construct living breakwaters along shorelines facing rapid erosion. Through the American Recovery and Reinvestment Act (ARRA), federal “stimulus funding” has been used to construct more than two kilometers of submerged breakwaters, which are designed to support the rapid settlement of oysters and so gradually morph into living reefs.
Design is a key element—the reefs must serve a function: protecting adjacent marshes and seagrass beds from their prior state of rapid erosion, and so also defending roads and valuable coastal property. One of the important criteria for this project was the creation of jobs—short-term employment during the design and construction phase, but more beside that. Economists have found other returns on the investment in the fisheries sector. Recent research shows that for every new hectare of oyster reef created in Mobile Bay, we can expect an additional 3,200 adult blue crabs every year.
Natural Coastal Protection
The drama, beauty, and economic viability of coastlines make them highly coveted places to live. But it also exposes human coastal settlements to storm surges, inundation, and erosion. All of these threats are exacerbated by the way people have engineered the coast, altered the flow of rivers, and changed sediment supplies.
The cost of natural hazards to coastal communities has increased substantially over recent decades. The proportion of the world’s GDP annually exposed to tropical cyclones has increased from 3.6% in the 1970s to 4.3% in the first decade of the 2000s. Billions of dollars are invested in reducing risks from coastal hazards and climate change, creating both threats and opportunities for natural systems. Most of those dollars are being spent on infrastructure like sea walls, jetties, and levee systems, yet natural coastlines can help buffer these threats, making them more important than ever to coastal communities.
Because of the role they play in mitigating the threat of storms, coastal wetlands, and reefs (both coral and oyster) have become part of the coastal defense strategy in some areas, which has led to active restoration of these habitats.
Three key processes
Coastal ecosystems play a remarkable role in shaping the physical structure of our coastlines and, in so doing provide critical services to people in reducing the physical impacts of erosion, storm-damage and flooding. These ecosystems support three key processes: wave attenuation, storm surge reduction and shoreline elevation.
- Wave attenuation: Wind and swell waves scour the coast and can drive erosion and the shifting of sediments. Larger waves can overtop beaches, dunes and artificial barriers such as seawalls causing flooding. Coastal ecosystems have a complex tangle of shoots, roots, shells or coral skeletons that cause friction, rapidly diminishing waves’ energy.
- Storm surge attenuation: Major storms and typhoons typically create a storm surge, a rise in the water level along tens or hundreds of kilometers of coastline and the end-result can be devastating floods. During storms, coastal ecosystems continue to reduce incoming overlying wind and swell waves, but where they are sufficiently extensive they also provide resistance to the landward flow of the storm surge itself. This way, even a partial reduction in surge heights can prevent large areas of flooding.
- Maintaining shoreline elevation: On average, sea levels are now rising by over 3 mm per year, with considerable local variation. Such rates are set to continue or increase over coming decades. While no human engineering can alter this
fact, many coastal ecosystems have a capacity to “grow” vertically, raising the elevation of the seabed or land on which they are growing. Coral reefs, oyster reefs, salt marshes, and mangroves have all been shown to be able to keep up with rising sea levels. Such processes are not guaranteed, as they can be countered by other natural processes of erosion or natural subsidence, but in at least some places they can may a remarkable difference. Reefs, mangroves, marshes and seagrass meadows can become dynamic self-maintaining barriers and coastal defenses.
Top image: ©Ethan Daniels. . Photo Credits in Text: © Jonathan Kerr, © Marjo Aho, © Mark Spalding, © Daniel White. Tiles (left to right): © Jeff Yonover, © Tim Calver, © Donna Squire, © Clay Bolt